Physics Ideas

Solar system: planets, characteristics, origin, evolution

The solar system is a set of planets and astronomical objects linked by gravitational attraction that produces the only central star: the sun. Within this planetary system, there are a multitude of smaller bodies such as moons, dwarf planets, asteroids, meteoroids, centaurs, comets or cosmic dust.

The solar system is 4.568 million years old and is located in the Milky Way. If you start counting from Pluto’s orbit, it is calculated to measure 5,913,520,000 km, equivalent to 39.5 AU.

The closest known planetary system is Alpha Centauri, located about 4.37 light-years (41.3 trillion kilometers) from our Sun. In turn, the closest star would be Proxima Centauri (probably from the Alpha Centauri system) , located approximately 4.22 light years.


The Sun is the largest and most massive object in the entire solar system, measuring no less than 2 x 10 30 kg and a diameter of 1.4 x 10 6 km. A million lands fit freely inside.

Analysis of sunlight shows that this huge sphere is composed mainly of hydrogen and helium, plus 2% of other heavier elements.

Inside, there is a fusion reactor, which constantly turns hydrogen into helium, producing the light and heat that it radiates.

The Sun and the other members of the solar system probably originated at the same time, from the condensation of an original nebula of matter, at least 4.6 billion years ago. The subject in this nebula may well have come from the explosion of one or more supernovae.

Although the Sun is not the biggest or brightest star, it is the most important star for the planet and solar system. It is a medium-sized star, quite stable and still young, located in one of the spiral arms of the Milky Way. Pretty common in general, but luckily for life on Earth. 

With its powerful gravitational force, the Sun makes possible the surprising variety of scenarios on each of the planets in the solar system, as it is the source of its energy through which it maintains the cohesion of its members.

What planets make up the solar system?

There are 8 planets in the solar system, classified into inner and outer planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. 

inner planets

The inner planets are Mercury, Venus, Earth and Mars. They are small, rocky planets, while outer planets like Jupiter are gas giants. This difference in density is due to the way the matter in the original nebula condensed. The further away from the Sun, the temperature decreases and therefore matter can form different compounds.

In the vicinity of the Sun, where the temperature was higher, only heavy elements and compounds, such as metals and silicates, were able to slowly condense and form solid particles. That’s how the dense planets emerged: Mercury, Venus, Earth and Mars.

outer planets

The outer planets are Jupiter, Saturn, Uranus and Neptune. They formed in more remote regions, where matter quickly condensed into ice. The rapid growth of these ice accumulations gave rise to objects of enormous size. However, inside these gigantic planets are not frozen, in fact they still radiate a lot of heat into space.

The boundary between the inner and outer planets is the Asteroid Belt, remnants of a planet that did not form due to Jupiter’s massive gravitational pull that spread them.

Is Pluto a planet in the solar system?

Pluto was considered a planet for a long time until 2006, when astronomers designated it as a dwarf planet for lack of orbital dominance, one of the characteristics that a celestial body must have to be considered a planet. 

This means that other bodies of similar size and gravity must not exist in your environment. This is not the case for Pluto, whose size is similar to that of its moon, Charon, and very close together.

Main features of planets

Planets orbit the Sun in elliptical orbits, according to Kepler’s laws. These orbits are all located on approximately the same plane, which is the plane of the ecliptic, in which the Earth’s movement around the Sun takes place.

In fact, almost all objects in the solar system are in this plane, with minor differences, except Pluto, whose orbital plane is tilted 17º relative to the ecliptic.

– Mercury

It is a small planet, a little bigger than a third of the Earth and closer to the Sun. On its surface, rock formations similar to those of the Moon can be seen, as seen in the images. Typical are the lobed cliffs , which astronomers say are an indication that Mercury is shrinking.

It also has other features in common with our satellite, for example its chemical composition, the presence of ice at the poles and a large number of impact craters.

Occasionally, Mercury is visible from Earth, far below the horizon, just at sunset or very early before sunrise.

This small planet has engaged its rotation and translation movement around the Sun, thanks to the so-called tidal forces. These forces tend to slow the planet’s rotation around its axis until it equals its translational speed.

Such couplings are not uncommon between objects in the solar system. For example, the Moon has a similar motion and always shows the same face of the Earth as Pluto and its satellite Charon.

Tidal coupling is responsible for Mercury’s extreme temperatures, along with the planet’s sparse atmosphere. 

The face of Mercury exposed to the Sun has scorching temperatures, but it is not the hottest planet in the solar system, although it is the closest to the star. This distinction is for Venus, whose surface is covered by a dense cloudiness that traps heat within.

Related:   Orion Nebula: origin, location, characteristics and data

Table 1. Mercury: characteristics and movement

– Venus

In size, mass and chemical composition, Venus is very similar to Earth, but its dense atmosphere prevents heat from escaping. It is the famous greenhouse effect, whose cause is that the surface temperature of Venus reaches 400 °C, close to the melting point of lead.

The Venusian atmosphere is mainly composed of carbon dioxide and traces of other gases, such as oxygen. Atmospheric pressure is about 100 times greater than on land and the distribution of fast winds is extremely complex.

Another detail of Venus’ remarkable atmosphere is its rotation around the planet, which takes about 4 days on Earth. Note that the planet’s own rotation is extremely slow: a Venusian day lasts 243 days on Earth.

Deuterium is abundant on Venus, a hydrogen isotope that is due to the lack of a protective ozone layer against the Sun’s ultraviolet rays. Currently, there is no evidence of water, however, as much deuterium indicates that Venus may have it on atmosphere. past.

As for the surface, radar maps show geographic features such as mountains, plains and craters, in which basalt is abundant.

Volcanism is characteristic of Venus, as is slow retrograde rotation. Only Venus and Uranus rotate in the opposite direction to the other planets. 

The hypothesis is that it is due to a collision in the past with another celestial object, but another possibility is that the atmospheric tides caused by the Sun slowly change the rotation. Possibly both causes contributed equally to the movement that the planet now has.

Table 2. Venus: characteristics and movement.

– The land

The third planet close to the Sun is the only one that harbors life, at least as far as we know.

The Earth is at an ideal distance for the proliferation of life and also has a protective layer of ozone, abundant liquid water (up to 75% of the surface is covered by this element) and its own intense magnetic field. Its rotation is also the fastest of the four rocky planets.

The Earth’s atmosphere is composed of nitrogen and oxygen, with traces of other gases. It is stratified, but its limits are not defined: it gradually diminishes until it disappears.

Another important feature of the Earth is that it has plate tectonics, so its surface undergoes continual changes (in geological times, of course). Therefore, the evidence of craters that abound on other planets in the solar system has already been erased.

This provides the Earth with a wide variety of environmental scenarios: mountains, plains and deserts, along with an abundance of water, both in the vast oceans and in fresh water on the surface and underground.

Along with the Moon, their natural satellite, they form a remarkable pair. The size of our satellite is relatively large compared to Earth’s and has a remarkable influence on it.

To begin with, the Moon is responsible for the tides, which exert a powerful influence on terrestrial life. The Moon is in synchronous rotation with our planet: its rotation and translation periods around the Earth are the same, that’s why it always shows us the same face.

Table 3. The Earth: characteristics and movement

– Mars

Mars is slightly smaller than Earth and Venus, but larger than Mercury. Its surface density is also slightly lower. Much like Earth, the curious always believed they saw signs of intelligent life in the reddish star.

For example, as early as the mid-19th century, many observers claimed to have seen “channels,” straight lines that crisscrossed the Martian surface and blamed the presence of intelligent life. Even maps of these supposed channels were created. 

However, images from the Mariner spacecraft showed, in the mid-sixties of the 20th century, that the Martian surface is desert and that the channels were non-existent. 

The reddish color of Mars is due to the abundance of iron oxides on the surface. As for its atmosphere, it is thin and consists of 95% carbon dioxide, with traces of other elements such as argon. There is no water vapor or oxygen. The latter is found to form compounds in rocks.

Unlike Earth, Mars does not have its own magnetic field; therefore, particles from the solar wind directly affect the surface poorly protected by the thin atmosphere. 

As for the orography, it is varied and there are indications that the planet once had liquid water. One of the most notable features is Mount Olympus, the largest known volcano in the Solar System so far.

Mount Olympus by far surpasses the largest volcanoes on Earth: it is three times as tall as Mount Everest and 100 times the volume of Mauna Loa, the largest volcano on earth. With no tectonic activity and with low gravity, lava could accumulate to give rise to such a colossal structure.

Table 4. Mars: characteristics and movement

– Jupiter

It is undoubtedly the king of the planets due to its large size: its diameter is 11 times that of Earth and its conditions are much more extreme.

It has a rich atmosphere crossed by fast winds. The well-known Great Red Spot of Jupiter is a longstanding storm, with winds of up to 600 km/h.

Jupiter is gaseous, so there is no solid ground under the atmosphere. What happens is that the atmosphere becomes denser as the depth increases, until it reaches a point where the gas is liquefied. Therefore, it is quite flat at the poles, due to rotation.

Related:   Pluto (planet): characteristics, composition, orbit, movement

Despite the fact that most of the matter that makes up Jupiter is hydrogen and helium – like the Sun -, inside it has a nucleus of heavy elements at a high temperature. In fact, the gas giant is a source of infrared radiation, so astronomers know that the interior is much hotter than the exterior. 

Jupiter also has its own magnetic field, 14 times stronger than Earth’s. A notable feature of this planet is the large number of natural satellites it has.

Due to its enormous size, it is natural that its gravity has captured many rock bodies that managed to pass through its vicinity. But it also has large moons, the most notable being the four Galilean moons: Io, Europa, Callisto and Ganymede, the last largest of the solar system’s moons.

These large moons probably originated at the same time as Jupiter. They are fascinating worlds, as they have the presence of water, volcanism, extreme weather and magnetism, among other characteristics.

Table 5. Jupiter: characteristics and movement

– Saturn

Without a doubt, what strikes Saturn most is its complex ring system, discovered by Galileo in 1609. It should also be noted that Christian Huygens was the first to notice the annular structure a few years later, in 1659. Galileo’s telescope did not have enough resolution.

Millions of ice particles make up Saturn’s rings, perhaps the remnants of ancient moons and comets that reach the planet – Saturn has about the same number as Jupiter. 

Some of Saturn’s satellites, called shepherd’s satellites , are responsible for keeping the orbit free and confining the rings to well-defined regions of the planetary equatorial plane. The planet’s equator is quite pronounced, being a very flat spheroid due to its low density and rotational movement.

Saturn is so light it could float in a hypothetical ocean big enough to contain it. Another reason for the planet’s deformation is that its rotation is not constant, but depends on latitude and other interactions with its satellites.

Regarding its internal structure, the data collected by the Voyager, Cassini and Ulysses missions ensure that it is quite similar to that of Jupiter, that is, a gaseous mantle and a core of very hot heavy elements.

The temperature and pressure conditions allow the formation of metallic liquid hydrogen, so that the planet has its own magnetic field.

On the surface, the weather is extreme: storms are plentiful, though not as persistent as those on neighboring Jupiter.

Table 6. Saturn: characteristics and movement

– Uranus

It was discovered by William Herschel in 1781, who described it as a small blue-green dot in his telescope. At first he thought it was a comet, but soon he and other astronomers realized it was a planet, just like Saturn and Jupiter.

Uranus’ movement is quite peculiar, being retrograde rotation, just like Venus. Furthermore, the axis of rotation is very tilted in relation to the plane of the orbit: 97.9º, so that it practically rotates laterally.

Therefore, the planet’s seasons – revealed by Voyager’s images – are quite extreme, with winters lasting 21 years.

Uranus’ blue-green color is due to the atmosphere’s methane content, much cooler than that of Saturn or Jupiter. But little is known about its internal structure. Uranus and Neptune are considered ice worlds, or rather gaseous or almost liquid.

Although Uranus does not produce metallic hydrogen because of its lower mass and internal pressure, it does have an intense magnetic field, roughly comparable to Earth’s.

Uranus has its own ring system, though it’s not as magnificent as Saturn’s. They are very weak and that is why they are not easily observed from Earth. They were discovered in 1977, thanks to the planet’s temporary hiding by a star, which allowed astronomers to see its structure for the first time.

Like all outer planets, Uranus has many moons. The main ones are Oberon, Titania, Umbriel, Ariel and Miranda, names taken from the works of Alexander Pope and William Shakespeare. Frozen water has been detected on these moons.

Table 7. Uranus: characteristics and movement

– Neptune

At the far end of the solar system is Neptune, the planet farthest from the Sun. It was discovered due to inexplicable gravitational disturbances, which predicted the existence of a large, as-yet-undiscovered object. 

The calculations of the French astronomer Urbain Jean Leverrier finally led to the discovery of Neptune in 1846, although Galileo had already seen it with his telescope, believing it to be a star.

Viewed from Earth, Neptune is a small blue-green dot and until recently little was known about its structure. The Voyager mission provided new data in the late 1980s.

The images showed a surface with evidence of strong storms and strong winds, including a large Jupiter-like spot: the Great Dark Spot.

Neptune has a methane-rich atmosphere and a weak ring system similar to Uranus. Its internal structure is composed of an ice crust that covers the metal’s core and has its own magnetism.

As for the moons, about 15 have been discovered so far, but there could be others, as the planet is very far away and is the least studied so far. Triton and Nereid are the main ones, with Triton in retrograde orbit and having a tenuous atmosphere of nitrogen.

Table 8. Neptune: characteristics and movement

other astronomical objects

The Sun and the large planets are the largest members of the solar system, but there are other objects, smaller but equally fascinating.

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We are talking about dwarf planets, moons or satellites of major planets, comets, asteroids and meteoroids. Each has extremely interesting quirks.

tiny planets

In the asteroid belt that lies between Mars and Jupiter, and beyond the orbit of Neptune, in the Kuiper belt, there are many objects that, according to astronomical criteria, do not fall into the category of planets.

The most prominent are:

– Ceres, in the asteroid belt.

– Pluto, formerly considered the ninth largest planet.

– Eris, discovered in 2003 and bigger than Pluto and farther from the Sun than that.

– Makemake, in the Kuiper belt and about half the size of Pluto.

– Haumea, also in the Kuiper belt. It has a markedly ellipsoidal shape and has rings.

The criterion for distinguishing them from the main planets is their size and gravitational attraction, linked to their mass. To be considered a planet, an object must revolve around the Sun, in addition to being more or less spherical.

And its gravity must be high enough to absorb other smaller bodies around it, like satellites or as part of the planet.

As at least the gravitational criterion is not met for Ceres, Pluto and Eris, this new category was created for them, which Pluto ended in 2006. In the distant Kuiper belt, it is possible that there are more dwarf planets like these, as yet undetected.


As we’ve seen, the major planets, and even Pluto, have satellites that orbit around them. There are more than one hundred belonging to the main planets, almost all of them distributed in the outer planets and three belonging to the inner planets: Moon of Earth, Phobos and Deimos of Mars.

There may still be more moons to discover, especially on planets farther away from the Sun, such as Neptune and other frozen giants.

Its shapes are varied, some spheroidal and others quite irregular. The biggest ones probably formed next to the parent planet, but others could be captured by gravity. There are even temporary moons, which for some reason are captured by the planet, but are released at the same time.

Other bodies, in addition to the main planets, also have moons. It is estimated that so far there are around 400 natural satellites of all types.


Comets are residues of the cloud of matter that gave rise to the solar system. They are made of ice, rocks and dust and are currently found around the solar system, although they occasionally get close to the sun.

There are three regions far from the Sun, but they still belong to the solar system. Astronomers believe that all comets live there: the Kuiper belt, the Oort cloud, and the scattered disk.

Asteroids, Centaurs and Meteoroids

Asteroids are rock bodies smaller than a dwarf planet or satellite. Most of them are found in the asteroid belt that marks the boundary on rocky and gaseous planets.

Centaurs, in turn, receive this name because they share characteristics of asteroids and comets, as well as mythological beings of the same name: half human and half horse.

Discovered in 1977, they have not yet been properly photographed, but are known to be abundant between the orbits of Jupiter and Neptune.

Finally, a meteoroid is a fragment of a larger object, like the ones described so far. They can be as tiny as a slab of matter – without being as small as a speck of dust – around 100 microns or up to 50 km in diameter.

Summary of main features of the solar system

– Estimated age : 4.6 billion years.
– Shape : disc
– Location : the Orion arm in the Milky Way.
– Extent : it is relative, it can be considered around 10,000 astronomical units *, up to the center of the Oort cloud.
– Types of planets : terrestrial (rocky) and Jovian (gas and ice)
– Other objects : satellites, dwarf planets, asteroids.

* An astronomical unit is equivalent to 150 million kilometers.

Origin and evolution

Currently, most scientists believe that the origin of the solar system lies in the remnants of one or more supernovae, from which a gigantic nebula of cosmic gas and dust formed.

Gravity was responsible for clustering and collapsing this matter, which in this way began to rotate faster and faster and form a disk, in the center of which the Sun formed. This process is called adding.

Around the Sun remained the remaining disk of matter, from which the planets and other members of the solar system emerged.

By observing star systems in the formation of our own Milky Way galaxy and computer simulations, scientists have evidence that these processes are relatively common. Newly formed stars often have these disks of matter around them.

This theory explains very well the discoveries made about our solar system, being a single central star system. However, this would not fully explain the formation of the planet in binary systems. And there are, as it is estimated that 50% of exoplanets belong to two-star systems, being very common in the galaxy.

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